feat(solo/stdlib): date_today() -> ISO YYYY-MM-DD (UTC)

[hyperpolymath]

Third and final Phase-2 stdlib primitive on tracker #45 (Map #49 → JSON #53 → date, this PR).

What

date_today() -> String — current UTC date as ISO YYYY-MM-DD. time() only gave a Unix timestamp float, so there was no way to stamp generated artifacts with a calendar date.

How

Civil date derived from SystemTime epoch seconds via Howard Hinnant's civil_from_days (public domain) — no date-crate dependency, correct for all Gregorian dates. div_euclid handles the (irrelevant-in-practice but correct) pre-epoch case.

Validation

• examples/date.my prints 2026-05-18 (matches today — algorithm verified end-to-end).
• test_eval_date_today asserts format/shape + sane ranges (exact value is clock-dependent). Full suite 16 passed, 0 failed.
• clippy-clean; new code rustfmt-clean (pre-existing stdlib.rs fmt drift untouched, consistent with feat(solo/stdlib): string-keyed Map/dict builtins #49/feat(solo/stdlib): json_parse / json_stringify (+ fix string-literal escapes) #53).

Scope

date_now_iso8601() (mentioned as optional in #45) intentionally deferred — not requested by #48, keeping this PR to one focused primitive. vscode-extension/package-lock.json (pre-existing untracked) not included.

Refs #45
Refs #48

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 35 issues detected

Severity	Count
🔴 Critical	6
🟠 High	11
🟡 Medium	18

⚠️ Action Required: Critical security issues found!

View findings

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